Radio communication systems such as LTE (Long Term Evolution) and LTE-A (LTE-Advanced) standardized in 3GPP (3rd Generation Partnership Project) are based on the premise that a plurality of radio base stations (hereinafter, referred to as base stations) are deployed, and each base station performs communication with radio terminals (hereinafter, referred to as terminals) in the communication area of the own station. This communication area is referred to as a cell, and a cell can also be divided into a plurality of areas by allowing antennas to have directivity. These divided areas are referred to as sector cells, and it is assumed hereinafter that simply stating “cell” refers to “sector cell”.
In recent years, with the proliferation of smartphones and the like, the traffic volume of data communication is increasing acceleratedly. To cope with such a situation, it is essential not only to introduce radio communication systems of high spectral efficiency such as LTE, but also to introduce a large number of small cell base stations of low transmission power in addition to macrocell base stations covering wider areas. A network in which cells of such various sizes coexist is referred to as a heterogeneous network and attracts more and more attentions recently.
In LTE, generally, the same radio frequency band (hereinafter, referred to as “radio band”) is used among neighbor cells. Accordingly, inter-cell interference may occur when neighbor cells use the same frequency band for transmission, regardless of uplink transmission or downlink transmission. In a heterogeneous network in particular, the problem of load balancing is also among concerns, in addition to the problem of interference caused along with the increasing number of cells. The problem of load balancing is that in a heterogeneous network, since cells of various sizes coexist and terminals distribute and congest in various ways, loads among cells become uneven, and so loads concentrate on a specific cell, resulting in the decreased communication rates of terminals in that cell.
In LTE, control of the uplink transmission power of a terminal is considered to be one of effective solutions to the above-described problems of interference and load balancing. Specifically, transmission power can be greatly adjusted for each terminal by controlling parameters related to target received power and propagation loss between a terminal and a base station (hereinafter, referred to as path loss), whereby avoidance of interference and improvement in channel quality can be achieved. Hereinafter, uplink transmission power control in LTE will be described briefly.
The transmission power PPUSCH of a PUSCH (Physical Uplink Shared CHannel), which is a channel for transmitting data of a terminal i, is defined as the following equation (1) (NPL 1):PPUSCHN=min[PCMAX(i),10 log10(MPUSCHN)+PO_PUSCH+α·PL+ΔTF(i)+f(i)]   (1)where PCMAX(i) [dBm] is the maximum transmission power of the terminal i, MPUSCH(i) is the number of resource blocks (RB) allocated to the PUSCH, PO_PUSCH [dBm] is target received power, PL [dB] is a path loss between the terminal and base station estimated from a downlink, a is a path loss correction coefficient, ΔTF(i) [dB] is a MCS (Modulation and Coding Schemes)-related parameter depending on deltaMCS-Enabled, which is notified from an upper layer, and f(i) [dBm] is a cumulative value of Closed Loop TPC (Transmission Power Control) correction coefficient δPUSCH. A resource block RB is a unit of radio band assignment and is also referred to as a physical resource block (PRB).
Further, the above-mentioned target received power PO_PUSCH is composed of two terms, as represented by the following equation (2):PO_PUSCH=PO_NOMINAL_PUSCH+PO_UE_PUSCH(i)  (2)where PO_NOMINAL_PUSCH [dBm] is target received power that is common among terminals connected to the same cell, and PO_UE_PUSCH(i) [dB] is a received power offset for each individual terminal. It is known that in the above equations (1) and (2), those greatly affecting an increase or a decrease in transmission power per resource block RB are a term related to the target received power (PO_PUSCH) and a term related to the path loss (α·PL).
PTL 1 proposes a solution to the above-described problems of interference and load balancing. According to PTL 1, when interference information OI (Over Load Indicator) and load information indicating a traffic rate are received from an adjacent cell, a base station adjusts the interference information by using the received load information and controls the uplink transmission power of a terminal in its own cell based on the adjusted interference information. For example, when the load on an adjacent cell is small, the received interference information is adjusted as if interference power is small, whereby it is possible to prevent a more decrease than necessary in transmission power. Note that the interference information OI is indication information that notifies alarm to an adjacent cell in PRB units when interference is measured (see NPL 2).